This invention relates to an electric circuit breaker, and more particularly to a vacuum circuit breaker having means for mechanically imposing a predetermined delay in the tripping operation thereof.
Vacuum circuit breakers are well known devices. Such vacuum circuit breakers generally include one or more vacuum interrupter modules. The maximum fault current interruption rating of such vacuum circuit breakers is related to the peak amplitude of the arcing fault current. In general, the amplitude of the offset, i.e., non-symmetrical, fault current is related to the time interval between the initiation of the fault and the parting of the circuit breaker contacts. That is, the longer the time interval, the lower the arcing fault current, and hence, the easier the interruption.
Inherently, the vacuum interrupter lends itself to fast interruption compared to air-magnetic and oil interruption devices. Hence, when a vacuum interrupter is used in conjunction with conventional circuit breaker operating mechanisms operating with conventional trip devices, it has been found that the tripping performance corresonds to about a three cycle time interruption rating. It is to be appreciated that the opening time rating of a circuit breaker is defined by ANSI rating standards in terms of a specified time from trip command to the parting of the circuit breaker contacts. For example, see ANSI C37.03, pages 9, 10, 1969. Such time ratings are generally stated in terms of a number of cycles at 60 Hertz. For those applications for which a five cycle interruption time rating is adequate, significant benefits can be derived from delaying the unnecessarilyearly parting of the circuit breaker contacts. For example, by delaying the parting of the contacts so as to meet the ANSI standard for a five cycle rating, a given circuit breaker will provide increased fault current interrupting capacity. Also, as compared to an unnecessarily rapid interruption, a delayed interruption may allow the user to employ a less costly vacuum interrupter to achieve the specified fault current rating.
Notwithstanding the advantages of delayed interruption, there are still applications in which more rapid fault interruption, e.g., three cycles, is required. Thus, it would be desirable to provide vacuum circuit breaker tripping means simply adaptable to meet standards for either three cycle or five cycle interruption ratings. It is particularly desirable to provide such adaptability through modifications of only the tripping mechanism, the rest of the circuit breaker assembly remaining unchanged.
In order to provide such delayed tripping means, several requirements must be satisfied. One such requirement is that the delayed tripping means must provide a relatively long tripping time which is not adversely affected by the wide range of circuit breaker operating conditions. For example, the delayed tripping means must operate at one of several nominal voltages, e.g., 48, 125, or 250 V dc with a specified trip time. It is necessary that, at each nominal voltage, the tripping performance must be essentially identical even though the design change is limited solely to changes in the tripping solenoid coil appropriate for the particular voltage. In all other respects, the trip device must remain unchanged. Further, at each of these nominal voltages, the trip time variations must not be excessive as the voltage varies between 75% and 125% of the nominal value. Similarly, the minimum and maximum allowable current which a circuit breaker trip coil can draw are usually restricted by other system considerations. In addition, the delayed tripping means must operate consistently with minimal variation in trip time within an ambient temperature range between about -30.degree. F. and about 140.degree. F. Also, the delayed tripping means, in addition to being simple and reliable, should be compact and insensitive to orientation. For example, for some special applications, it may be necessary to provide two redundant delayed tripping means for each vacuum interrupter in a limited space. In such applications, it is often only possible to install such redundant delayed tripping means in the availble limited space by orienting the two tripping means at different angles with respect to a horizontal plane. Thus, gravitational effects are different in the two delayed tripping means. In addition, it is desirable that the delayed tripping means be simply adaptable to allow the tripping time to be selectably chosen, e.g., to provide either three cycle or five cycle interrupting time rating.
The above-described requirements are not all satisfied by available techniques in which the delay is achieved through such means as high inductance coils, fluid dashpots, and multiple latch devices.
Accordingly, it is a general object of this invention to provide a vacuum circuit breaker with delayed tripping means.
It is another object of this invention to provide such delayed tripping means which is substantially unaffected by circuit breaker operating conditions.
It is another object of this invention to provide such delayed tripping means which is simply adaptable to allow the tripping time to be selectably chosen.
It is another object of this invention to provide such delayed tripping means having a five cycle interrupting time rating.
It is another object of this invention to provide such delayed tripping means in a redundant configuration.